1. Field of the Invention
The present invention relates to wireless communication systems, and more particularly, to a method and a base station for providing phase-sweep transmit diversity (PSTD) in a wireless communication system.
2. Description of Related Art
A wireless communication system is a complex network of systems and elements. Typically elements include (1) a radio link to a mobile station (e.g., cellular telephone, PDAs, computers, etc.), which is usually provided by at least one and typically several base stations, (2) communication links among base stations, (3) a controller, typically one or more base station controllers or centralized base station controllers (BSC/CBSC), to control communication and to manage the operation and interaction of the base stations, (4) a controller (e.g., mobile switching center (MSC)) or switch, for routing calls and/or data within the system, and (5) a link to the land line or public switch telephone network (PSTN).
One aspect of designing a wireless communication system is to improve the performance of forward link or downlink transmissions. That is, the voice and packet data transmissions from a base station to a mobile station. However, multipath fading may cause multiple copies of the transmissions to be received at the mobile station with time-varying attenuation, phase shift and delay because of multiple reflections on the path.
One technique to mitigate the effects of multipath fading in a wireless communication channel is error correcting codes. Combined with a bit interleaving scheme, an error correction code can reduce bit errors caused by multipath fading. In particular, bit interleaving scatters the bit errors among the uncorrupted bits (i.e., “good” bits) so that the error correction codes can better correct the error bits. However, the fading process must be fast enough so that a burst of bit errors is much shorter than the bit interleaving period (i.e., a frame) for bit interleaving to be effective. For example, a slow moving mobile station (e.g., a mobile station used by a pedestrian or an in-building user) creates a slow fading process such that fading bursts on the wireless communication channel are longer than a frame. As a result, the error correction code may not always compensate for the error bits.
Diversity is another technique used to reduce the effect of multipath fading. In particular, multiple antennas at the reception end, e.g., the mobile station, may be used to combine, select and/or switch between multipaths to improve the quality of the transmission from the transmission end, e.g., the base station. Forward link or downlink performance may be further improved by implementing diversity on the transmission end. In particular, phase-sweep transmit diversity (PSTD) may be implemented to reduce multipath fading effects. To provide PSTD, a base station generally includes a signal source, a frequency-shift element, a transmitting unit, and two or more transmitting antennas. A basic flow for providing PSTD may start with the signal source providing a baseband signal to a frequency-shift element. The frequency-shift element imposes time-varying phase shifts (equivalent to frequency shift) on the baseband signal and produces multiple signal streams where each signal stream corresponds to each transmitting antenna. The transmitting unit then modulates the frequency-shifted signals to produce radio frequency (RF) signals and amplifies the RF signals with power amplifiers. Each individual RF signal is then transmitted on each antenna. Typically, the time-varying phases imposed on antennas are symmetric. For an example of a two-antenna system, if Φ(t) is imposed on the first antenna at time t, −Φ(t) will be imposed on the second antenna at time t. The overall phase difference between the two antennas will be 2Φ(t) in this example. Furthermore, the antenna separation determines the transmission characteristic. If the antenna separation is large, the communication paths between each antenna and the receiver unit are more likely to be independent, and the antennas exhibit diversity characteristic. If the antenna separation is small, the transmitted signal forms energy-concentrated beams, which may sweep across the sector if time-varying phases are imposed on the antennas.
As will be appreciated, in PSTD the phase difference between the base station transmit antennas changes periodically. This period is referred to as the sweeping frequency. When base stations in the wireless network apply the same sweeping frequency (or integer multiples of a same sweeping frequency), the desired signal and interference may sweep at the same frequency (or integer multiple thereof). This may lower the signal-to-interference plus noise ratio (SINR) in some areas, and these areas may always stay in the low SINR condition as the desired and interference beams always sweep to these areas at the same time when sweeping frequencies are the same among neighboring sectors. For high speed data systems (e.g., CDMA2000 EVDO systems), a low SINR may mean low data throughput. For a voice system (e.g., a CDMA voice system), low SINR may cause increases in base station transmit power and this can reduce system capacity.